Tra từ once - Từ điển Oxford Advanced Learner 8th

once adverb, conjunction BrE [wʌns] NAmE [wʌns]
adverb
1. on one occasion only; one time
• I've only been there once.
• He cleans his car once a week.
•She only sees her parents once every six months.
• (informal)He only did it the once.
2. at some time in the past
• I once met your mother.
• He once lived in Zambia.
•This book was famous once, but nobody reads it today.
3. used in negative sentences and questions, and after if to mean ‘ever’ or ‘at all’
•He never once offered to help.
•If she once decides to do something, you won't change her mind.

Word Origin:
Middle English ones, genitive of ↑one. The spelling change in the 16th cent. was in order to retain the unvoiced sound of the final consonant.
Idioms: ↑all at once ▪ ↑at once ▪ ↑for once ▪ ↑going once, going twice, sold ▪ ↑just this once ▪ ↑once a …, always a … ▪ ↑once again ▪ ↑once and for all ▪ ↑once bitten, twice shy ▪ ↑once in a blue moon ▪ ↑once in a while ▪ ↑once more ▪ ↑once or twice ▪ ↑once too often ▪ ↑once upon a time

conjunction
as soon as; when
• We didn't know how we would cope once the money had gone.
• The water is fine once you're in!

Word Origin:
[once] Middle English ones, genitive of ↑one. The spelling change in the 16th cent. was in order to retain the unvoiced sound of the final consonant.

See also: ↑going, going, gone